Laser rangefinders

Abstract

A laser rangefinder comprises a transmitter (19) controlled by sequence-controlled pulses emitted by a timer (16) following actuation of a switch (17) by an operator. The receiver of the rangefinder comprises an avalanche photodiode (APD) (10) the sensitivity of which is controlled in a time-programmed manner by a bias voltage controller (11) which receives timing pulses from the timer (16). The time-programmed control operates to render the APD 10 insensitive to protect the receiver against optical element backscatter and atmospheric backscatter prior to rendering the APD 10 sensitive to monitor return pulses from remote targets. Optimum performance of the APD 10 is achieved by provision of a monitor circuit (24) which in combination with controller (11) establishes the breakdown voltage of the APD 10 at the temperature prevailing on each occasion immediately prior to firing of the laser transmitter (19) and derives therefrom the corresponding APD operating voltage to be achieved when the APD 10 next becomes sensitive for return pulse monitoring, this operating voltage being a fixed fraction of the previously established APD breakdown voltage. In one arrangement for implementing circuit (24) the APD breakdown voltage is monitored by a first capacitor (C6) which is arranged to function as a voltage source during the time-programmed operation and during this operation a second capacitor (C5) is connected in parallel with the APD 10 and in series with capacitor (C6) whereby the APD operating voltage is determined by capacitor (C5).

I claim: 
     
       1. A laser rangefinder having a transmitter and a receiver, wherein the receiver comprises an avalanche photodiode the bias voltage of which is provided by circuitry controlled in time from the laser transmitter to establish substantially zero bias voltage when the transmitter is fired and to increase the bias voltage during a predetermined time interval therafter to establish a final bias voltage substantially equal to a fixed percentage below the breakdown voltage of the photodiode prevailing immediately prior to firing of the laser transmitter, said circuitry incorporating means to determine said breakdown voltage at the operating temperature prevailing immediately prior to firing of the laser transmitter. 
     
     
       2. A laser rangefinder as claimed in claim 1, wherein said circuitry comprises a first voltage source having a level substantially in excess of said breakdown voltage and applicable to said photodiode immediately prior to firing of the laser transmitter to establish the breakdown voltage than prevailing. 
     
     
       3. A laser rangefinder as claimed in claim 2, wherein said determining means is arranged to function as a second voltage source for said photodiode during said predetermined time interval. 
     
     
       4. A laser rangefinder as claimed in claim 2, wherein said determining means comprises a first capacitor connected in parallel with said photodiode when said first voltage source is applied thereto for determining said breakdown voltage and said circuitry comprises a switching device which is controlled in time by the laser transmitter and which when actuated causes said first capacitor to function as a second voltage source, a second capacitor being connected in parallel with said photodiode by said switching device whereby the relative capacitive values of said first and second capacitors determine said percentage.